In various situations, there are demands for rapid, accurate, and convenient (that is, non-invasive) measurement of an internal temperature instead of a surface temperature of an object to be measured (hereinafter referred to as “measurement object”). Measurement of temperatures of biological bodies, including a human body, is a typical example of those demands. However, it is generally difficult to measure the internal temperature (sometimes referred to as deep/core body temperature, etc.) of the internal region of the biological body, that is, the temperature of the biological body in an internal region that is considered to be held substantially at a constant temperature due to blood flow. When the measurement object is a human body, holding a thermometer at a position where heat is not easily lost, such as under the tongue or arm, reading the thermometer after a thermal equilibrium state is attained between the thermometer and the human body, and then adopting the temperature read as the body temperature, is the common way often used. However, it takes a long time of about 5 minutes to 10 minutes to attain the thermal equilibrium state, and the obtained body temperature does not always match with the internal temperature of the human body. Therefore, it may be difficult to apply such a method to an object having difficulty in undergoing long-term body temperature measurement, such as babies and patients with a certain injury or disease. Further, it is difficult to obtain a body temperature with accuracy high enough for precise body temperature management.
In view of the above, as a thermometer for rapidly and accurately measuring the internal temperature of the human body, there has been proposed a thermometer including at least two sets of sensors each including a first temperature sensor that is brought into contact with the body surface, and a second temperature sensor arranged so as to oppose the first temperature sensor across a heat insulating member. In such a thermometer, a system of heat conduction equations is solved based on the temperature measurement results of the respective temperature sensors in a steady state to obtain the internal temperature. Therefore, the thermometer is designed so that magnitudes of heat fluxes passing through the respective sets differ from each other.
For example, in JP 2007-212407 A, there is disclosed a thermometer in which a thermal resistance value of the heat insulating member differs for each sensor set.
Further, in JP 4798280 B, there is disclosed a thermometer in which a heat insulating member is further arranged between the second temperature sensor (intermediate sensor) and outside air so that a thermal resistance value of the heat insulating member differs for each sensor set.
Further, in JP 2008-76144 A, there is disclosed a thermometer in which a heatsink having a different area is arranged between the second temperature sensor (temperature measuring means 21, 22) and outside air for each sensor set.